There is often a need for a power supply circuit that is capable of delivering power with high frequency components (fast, dynamically changing voltage and current), at high overall power conversion efficiency. For example, a radio frequency (RF) power amplifier (PA), i.e., an RF PA, can be fed by an efficient power supply at a reduced voltage, allowing the PA to operate more efficiently (i.e., with lower power consumption).
In an envelope tracking system, the power supply feeds the PA with a variable voltage that tracks the output power envelope of the PA. This provides for a reduced voltage while still maintaining enough operating headroom for the PA's output stage to prevent saturation. Note that the power supply must be capable of changing the output voltage very quickly to accommodate rapid changes in the output power envelope of the PA. At the same time, a high overall efficiency is desired in the power supply to achieve the desired lower power consumption.
A typical switched-mode power supply (SMPS) circuit achieves high efficiency. Unfortunately, it cannot deliver sufficiently high frequency components of the power, because the low switching frequencies commonly used in these types of regulators (a limitation largely imposed by the magnetic and switching losses) bound the regulator's bandwidth. Linear regulators, on the other hand, may be designed to deliver high frequency components, but the power conversion efficiency of such linear regulators is poor. Thus neither a common SMPS nor a linear regulator can meet this need.
Another example of the need for a power supply that is both efficient and can deliver a fast changing voltage and current is one that supplies a digital circuit, which may include a microprocessor. The digital circuit may operate more efficiently if fed by a power supply that adjusts its voltage dynamically to match the predicted processing needs.
Typically, the voltage is adjusted upward when the digital circuit is operating at high speeds, and downward when operating at lower speeds. While conventional power supplies can typically change their voltage within 50 ms, this delay may prevent the digital circuitry from operating at peak efficiency. A power supply that adjusts its voltage more quickly to allow for a more frequent change in clocking speeds of the digital circuitry is desirable.
Further, minimal or low voltage ripple is desirable in switching power supplies. For example, modern microprocessors are increasingly operated at low voltages due to increased chip density and lower voltage breakdown in advanced CMOS (Complementary Metal Oxide Semiconductor) technology. At these low voltages, the power supply ripple may be a substantial portion of the supply voltage. High ripple may undesirably require the power supply output voltage to be raised above the optimal level in order to ensure that the microprocessor is supplied with the minimal voltage required during periods when the ripple voltage drives the voltage excursions to a minimum. As an additional example, an RF PA requires its power supply to exhibit low ripple at its output. Ripple typically occurs synchronously with the switching frequency of the switching regulator and can feed through to the output of the PA, causing unwanted distortion in the RF output signal.
There have been some efforts to improve the conventional switching regulator circuits. For example, some prior art suggests the use of both a switching regulator and a linear regulator that feed a simple summing node to form the output of the power supply. The intention of such combination is for the linear regulator to provide the high frequency, and the switching regulator to provide the low frequency and DC components of the current to the load. These circuits, however, place a high burden on the linear regulator, as it requires the linear regulator to supply a large amount of excess current to modulate the voltage in the large reservoir capacitors needed by the switching regulator. Alternatively, a switching regulator and linear regulator may be placed in series, with the switching regulator's output feeding the linear regulator's input. In this arrangement, the linear regulator may be capable of delivering high frequency components of the power, while the switching regulator may deliver power efficiently to the linear regulator. However, this series arrangement forces all the power delivered to the load to pass through the linear regulator, causing power dissipation in the linear regulator and substantially reducing the overall efficiency of the power supply.
Therefore, there remains a need for a dynamic power supply system that has high overall efficiency, high bandwidth, and low voltage ripple.